You can use the Tbus, but coming back from int -> dom with QF, its much better to use the QF interterminal bus, as it drops you off in the secure dom zone, instead of having to do dom sec screening out the front, at T3.
Or if you are flying JQd after that QFi flight.
Going from dom -> int, if you have an overnighter, the T bus is good, or if you have a few hrs between your dom -> int flight.
Or if you have flown in with the 2nd cousin ie JQ from T2.
Tho, yes, the QF interterminal bus can be crowded, filled to the brim, but in a way tho, if you put up with the sardine can, you are dropped off in a better spot than out the front of T1.
If they (Tbus) drops you off at T1, you will still have to go upstairs to int dep to go through int sec screening and passport control, but the QF bus drops you off in that little corner area, that hasnt got every Tom/Karen/Harry/Dic*/Mary fighting for the spot to get to the various counters to check in.
If you get the QF interterminal bus, you would already have your BP, and can just go up that set of long escalators, and go behind the check in counters to int dep sec screening, and immi.
Lots of variables.